The present invention relates to a control system of ignition timing for internal combustion engines and more particularly to a method and system for ignition timing control of the engine. The control system controls the ignition timing of the engine at the time of fuel cutting with a time delay when engine speed is decelerating.
Therefore, in an automotive vehicle, it has been practiced to control an air-fuel ratio and the ignition timing by a feedback control to purify the exhaust gas. In order to save fuel and to protect a catalyst converter during deceleration of the vehicle, it has also been a practice to cut the fuel supply. If this fuel cut-off is carried out abruptly, the engine torque deviates easily from a correct value. By the inverse torque from the road, the engine torque becomes a negative torque, whereby an abrupt fluctuation occurs in the torque. As a consequence of this torque fluctuation, there has been a possibility of troubles at the time of deceleration of the vehicle speed such as intermittent abnormal vibrations or a sudden drop in the engine speed which causes a stall of the engine.
In order to prevent such a defective operation due to the fuel cut-off at the time of engine deceleration, a delay or retardation time is provided while the fuel is cut-off. Such a technique has been disclosed in, for example, Japanese Patent Laid Open Publication No. 170830/1983. By this known technique, the deceleration is detected on the basis of output signals of an engine rotation sensor and a throttle valve opening degree sensor. In accordance with the output signals of the two sensors, the start of the fuel cut-off is computed and controlled by a controller.
However, until the fuel is cut at the time of deceleration, the ignition timing remains unchanged. For this reason, insufficient combustion of the fuel occurs because of a delay in the ignition timing and causes a variation in an air-fuel ratio or a rise in intake negative pressure. As a consequence, there has been a problem of deterioration of the purification of the exhaust gas due to the emission of unburned gas.